Some computer systems may use adaptive power management policies to manage power and energy consumption. Managing power and energy is done by means of Dynamic Voltage and Frequency Scaling (DVFS). In this example of computer systems, as central processor unit (CPU) utilization decreases, the processor may transition to a lower performance state to conserve power. As the CPU utilization increases, the processor may transition to a higher performance state and may consume more power. An industry standard interface, called Advanced Configuration and Power Interface (ACPI), is defined for the purpose of controlling the frequency/voltage state of the processor.
In ACPI terminology, a frequency/voltage state is called performance state (P-state). Common implementations of P-state control are based on demand. In existing ACPI-based platforms, an operating system (OS) may load a table of performance state (P-state) information. An operating frequency of the processor is represented with corresponding control, status, and latency information. Furthermore, the OS may hold a utilization value for each state for transition to the next P-state up or down. The OS may manage the CPU P-states by directly controlling the calculated P-state at any given time.
For example, a range of predetermined P-states are provided to control the processor power consumption. As the CPU utilization decreases, the processor is transitioned to a lower predetermined P-state to conserve power. As the CPU utilization increases, the processor is transition to a higher predetermined P-state and may consume more power. In existing operating systems, the target P-state selection is based on the combination of processor utilization and the last selected P-state.
However, the OS have more visibility to user preferences, application type (such as real time requirements, visual quality demands, etc.) and does not response fast enough to changes in the workload of the processor caused by the hardware and a micro architecture of the processor.
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